1. Field of the Invention
This invention relates broadly to ophthalmic implants. More particularly, this invention relates to intraocular lenses which are focusable and allow for accommodation for near vision.
2. State of the Art
Referring to FIG. 1, the human eye 10 generally comprises a cornea 12, an iris 14, a ciliary body (muscle) 16, a capsular bag 18 having an anterior wall 20 and a posterior wall 22, and a natural crystalline lens 24 contained with the walls of the capsular bag. The capsular bag 18 is connected to the ciliary body 16 by means of a plurality of zonules 26 which are strands or fibers. The ciliary body 16 surrounds the capsular bag 18 and lens 24, defining an open space, the diameter of which depends upon the state (relaxed or contracted) of the ciliary body 16.
When the ciliary body 16 relaxes, the diameter of the opening increases, and the zonules 26 are pulled taut and exert a tensile force on the anterior and posterior walls 20, 22 of the capsular bag 18, tending to flatten it. As a consequence, the lens 24 is also flattened, thereby undergoing a decrease in focusing power. This is the condition for normal distance viewing. Thus, the emmetropic human eye is naturally focused on distant objects.
Through a process termed accommodation, the human eye can increase its focusing power and bring into focus objects at near. Accommodation is enabled by a change in shape of the lens 24. More particularly, when the ciliary body 16 contracts, the diameter of the opening is decreased thereby causing a compensatory relaxation of the zonules 26. This in turn removes or decreases the tension on the capsular bag 18, and allows the lens 24 to assume a more rounded or spherical shape. This rounded shape increases the focal power of the lens such that the lens focuses on objects at near.
As such, the process of accommodation is made more efficient by the interplay between stresses in the ciliary body and the lens. When the ciliary body relaxes and reduces its internal stress, there is a compensatory transfer of this stress into the body of the lens, which is then stretched away from its globular relaxed state into a more stressed elongated conformation for distance viewing. The opposite happens as accommodation occurs for near vision, where the stress is transferred from the elongated lens into the contracted ciliary body.
In this sense, referring to FIG. 2, there is conservation of potential energy (as measured by the stress or level of excitation) between the ciliary body and the crystalline lens from the point of complete ciliary body relaxation for distance vision through a continuum of states leading to full accommodation of the lens.
As humans age, there is a general loss of ability to accommodate, termed “presbyopia”, which eventually leaves the eye unable to focus on near objects. In addition, when cataract surgery is performed and the natural crystalline lens is replaced by an artificial intraocular lens, there is generally a complete loss of the ability to accommodate. This occurs because the active muscular process of accommodation involving the ciliary body is not translated into a change in focusing power of the implanted artificial intraocular lens.
There have been numerous attempts to achieve at least some useful degree of accommodation with an implanted intraocular lens which, for various reasons, fall short of being satisfactory. In U.S. Pat. No. 4,666,446 to Koziol et al., there is shown an intraocular lens having a complex shape for achieving a bi-focal result. The lens is held in place within the eye by haptics which are attached to the ciliary body. However, the implant requires the patient to wear spectacles for proper functioning. Another device shown in U.S. Pat. No. 4,944,082 to Richards et al., also utilizes a lens having regions of different focus, or a pair of compound lenses, which are held in place by haptics attached to the ciliary body. In this arrangement, contraction and relaxation of the ciliary muscle causes the haptics to move the lens or lenses, thereby altering the effective focal length. There are numerous other patented arrangements which utilize haptics connected to the ciliary body, or are otherwise coupled thereto, such as are shown in U.S. Pat. No. 4,932,966 to Christie et al., U.S. Pat. No. 4,888,012 to Home et al. and U.S. Pat. No. 4,892,543 to Turley, and rely upon the ciliary muscle to achieve the desired alteration in lens focus.
In any arrangement that is connected to the ciliary body, by haptic connection or otherwise, extensive erosion, scarring, and distortion of the ciliary body usually results. Such scarring and distortion leads to a disruption of the local architecture of the ciliary body and thus causes failure of the small forces to be transmitted to the intraocular lens. Thus, for a successful long-term implant, connection and fixation to the ciliary body is to be avoided if at all possible.
In U.S. Pat. No. 4,842,601 to Smith, there is shown an accommodating intraocular lens that is implanted into and floats within the capsular bag. The lens comprises front and rear flexible walls joined at their edges, which bear against the anterior and posterior inner surfaces of the capsular bag. Thus, when the zonules exert a tensional pull on the circumference of the capsular bag, the bag, and hence the intraocular lens, is flattened, thereby changing the effective power of refraction of the lens. The implantation procedure requires that the capsular bag be intact and undamaged and that the lens itself be dimensioned to remain in place within the bag without attachment thereto. Additionally, the lens must be assembled within the capsular bag and biasing means for imparting an initial shape to the lens must be activated within the capsular bag. Such an implantation is technically quite difficult and risks damaging the capsular bag, inasmuch as most of the operations involved take place with tools which invade the bag. In addition, the Smith arrangement relies upon pressure from the anterior and posterior walls of the capsular bag to deform the lens, which requires that the lens be extremely resilient and deformable. However, the more resilient and soft the lens elements, the more difficult assembly within the capsular bag becomes. Furthermore, fibrosis and stiffening of the capsular remnants following cataract surgery may make this approach problematic.
U.S. Pat. No. 6,197,059 to Cumming and U.S. Pat. No. 6,231,603 to Lang each disclose an intraocular lens design where the configuration of a hinged lens support ostensibly allows the intraocular lens to change axial position in response to accommodation and thus change effective optical power. U.S. Pat. No. 6,299,641 to Woods describes another intraocular lens that also increases effective focusing power as a result of a change in axial position during accommodation. In each of these intraocular lenses, a shift in axial position and an increase in distance from the retina results in a relative increase in focusing power. All lenses that depend upon a shift in the axial position of the lens to achieve some degree of accommodation are limited by the amount of excursion possible during accommodation.
U.S. Pat. No. 5,607,472 to Thompson describes a dual-lens design. Prior to implantation, the lens is stressed into a non-accommodative state with a gel forced into a circumferential expansion channel about the lens. At implantation, the surgeon must create a substantially perfectly round capsulorrhexis, and insert the lens therethrough. A ledge adjacent to the anterior flexible lens is then bonded 360° around (at the opening of the capsulorrhexis) by the surgeon to the anterior capsule to secure the lens in place. This approach has numerous drawbacks, a few of which follow. First, several aspects of the procedure are substantially difficult and not within the technical skill level of many eye surgeons. For example, creation of the desired round capsulorrhexis within the stated tolerance required is particularly difficult. Second, the bonding “ledge” may disrupt the optical image produced by the adjacent optic. Third, intraocular bonding requires a high degree of skill, and may fail if the capsulorrhexis is not 360° round. Fourth, the proposed method invites cautionary speculation as to the result should the glue fail to hold the lens in position in entirety or over a sectional region. Fifth, it is well known that after lens implantation surgery the capsular bag, upon healing, shrinks. Such shrinking can distort a lens glued to the bag in a pre-shrunk state, especially since the lens is permanently affixed to a structure which is not yet in equilibrium. Sixth, Thompson fails to provide a teaching as to how or when to release the gel from the expansion channel; i.e., remove the stress from the lens. If the gel is not removed, the lens will not accommodate. If the gel is removed during the procedure, the lens is only in a flattened non-accommodating shape during adhesion to the capsule, but not post-operatively, and it is believed that the lens therefore will fail to interact with the ciliary body as required to provide the desired accommodation as the capsular bag may change shape in the post-operative period. If the gel is otherwise removed thereafter, Thompson ostensibly requires an additional surgical procedure therefor. In view of these problems, it is doubtful that the lens system disclosed by Thompson can be successfully employed.
Thus, the prior art discloses numerous concepts for accommodating intraocular lenses. However, none are capable of providing an accommodating implant which does not, in one way or another, risk damage to the ciliary body or the capsular bag, present technical barriers, or present potential serious consequences upon failure of the device.